A mitochondrion is a cell organelle bearing an energy production characteristic in a eukaryotic cell, and supplies chemical energy (ATP) to a cell by an oxidative phosphorylation (OXPHOS). A mtDNA is a multicopy, circular, double-stranded DNA with 16.5 kb, and codes for 13 polypeptides, which are subunit proteins of 4 respiratory chain complexes essentially necessary for OXPHOS, 2 ribosomal RNAs (12S rRNA, and 16S rRNA) and 22 transfer RNAs (tRNAs), which are crucial for a mitochondrial protein synthesis.
Mitochondria supply 90% of energy necessary for a cell by OXPHOS in the form of ATP. Therefore, if a mitochondrial dysfunction happens, failures may ensue in central nerves, skeletal muscles, or cardiac muscles, which all have high demand for energy. In particular, a mitochondrial disease developed in central nerves or muscles is called mitochondrial myopathy, and classified into 3 disease-patterns on the basis of clinical conditions, i.e., mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (hereinafter referred to as MELAS), myoclonic epilepsy associated with ragged-red fibers (hereinafter referred to as MERRF), and chronic, progressive, external ophthalmoplegia (hereinafter referred to as CPEO).
In the above three disease-patterns, MELAS is a lethal mitochondrial genetic disease characterized by a stroke-like episode, hyperlactacidemia or the like, which highly frequently ensues in mitochondrial diseases. Pathogenetic point mutations which cause MELAS exist intensively in a mitochondrial tRNALeu(UUR) gene, and 80% of MELAS patients have a one-base substitution (hereinafter referred to as A3243G mutation) of adenine (A) to guanine (G) at mitochondrial base number 3243 on the mitochondrial tRNALeu(UUR) gene in mtDNAs (see FIG. 1). It is known that an A3243G mutation may cause various clinical symptoms including MELAS, such as mitochondrial diabetes, deafness, cardiomyopathy, or CPEO. In the research of molecular pathology, the A3243G mutation is a point mutation on which studies have advanced most. In the MELAS A3243G mutation mtDNAs and normal (wild) mtDNAs coexist in the same cell, and this condition is called heteroplasmy. MELAS is developed when a ratio of the A3243G mutation mtDNAs becomes in excess of 60 to 95% in a cell, which is called the threshold effect.
As a medicament for treating a mitochondrial genetic disease including MELAS, a pharmaceutical composition containing a pyrimidine nucleotide precursor and creatine as an active ingredient (Patent literature 1), a pharmaceutical composition containing 4-(p-quinolyl)-2-hydrozybutaneamide derivative as an active ingredient (Patent literature 2), a pharmaceutical composition containing alanine as an active ingredient (Patent literature 3), and so on, are known. However, the target of the known pharmaceutical compositions as above is not the gene mutations, which are the primary cause of MELAS, but the purposes thereof are merely symptomatic treatment for the conditions in central nerves or muscles. Thus, the effects thereof were limited.
As a treatment wherein a target is the gene mutation causing the mitochondrial genetic disease, a selective inhibition of replication of the MERRF mutation mtDNAs by binding peptide nucleic acids (hereinafter referred to as PNA) to an A8344G mutation which is the MERRF mutation was attempted (Non-patent literature 1). Specifically, effects on inhibition of replication of the A8344G mutation mtDNAs of MERRF in vitro by PNA in mtDNA replication run-off assay system, using PNA capable of binding to a sequence of a single-stranded H-chain existing in the MERRF A8344G mutation mtDNA under replication were assayed (Non-patent literature 1). In experiments therein, truncated mtDNAs, wherein synthesized extension was inhibited, were detected, and thus, inhibition of replication of the A8344G mutant mtDNAs by PNA was observed. However, when MERRF cybrid cells were cultured in a medium to which PNA was added, a shift of heteroplasmy to normal (wild-type) mtDNAs was not observed, namely, an effect of inhibition of replication of the A8344G mutation mtDNAs by PNA in living cells was not observed (Non-patent literature 2).